A standard prefilled hypodermic syringe comprises a tubular glass syringe body having a restricted front end that is provided with tip cap or needle. Around midway of its length the body is formed with a bypass passage constituted as a radially inwardly open and axially extending bypass groove. The interior of the syringe is subdivided by a front partition piston set just rearward of the bypass groove into a front compartment defined between the front partition piston and the front body end and a rear compartment bounded rearward by a rear piston fixed to a plunger that extends out a rear end of the body. A medicament, which may be a lyophilized powder or a liquid, is held in the front compartment and a solvent, typically water, is held in the rear compartment. For use the rear plunger is pressed forward so as to shift the front piston forward until it is level with the bypass passage which therefore opens into both the front and rear compartments, at which time the front piston stops moving while the liquid is forced by the rear piston through the bypass passage into the front compartment where it can mix with the medicament therein. Once the rear compartment is empty, the rear piston abuts the rear face of the front piston and the two move forward together to express the solution through the needle from the front compartment. The system can also be used with a medicament having two components that need to be mixed immediately before use.
Thus to make such a syringe it is necessary first to accurately set the front piston in place, then fill the rear compartment and set the rear piston and plunger in place, invert the assembly, and fill the medicament into the front compartment and install a tip cap or needle. Both of these operations--setting the rear piston in place and filling fluids into the compartments--are done by an apparatus which inserts a tube into the syringe body.
More particularly the front piston is set in place by compressing and fitting it into the front end of a setting tube whose outside diameter is slightly less than the inside diameter of the syringe body. This tube, with the piston fitted to its end, is inserted into the rear end of the piston until it is just rearward of where the piston should be set, then the piston is pressed forward out of the setting tube, whereupon its natural elasticity causes it to expand radially and set itself stably in place in the syringe body. Then the setting tube is withdrawn rearward.
To fill the fluid medicament or solid into one of the compartments, the syringe body is oriented with the compartment to be filled open upward and the filling apparatus inserts a filling tube down into the syringe. Once this tube is near the floor of the compartment, defined by the front face of the front piston, the medicament is fed out of the tube end. When dealing with a liquid, as when the apparatus is filling the rear compartment with solvent, it is standard to withdraw the filling tube upward as the liquid level rises in order to avoid turbulence.
In each of these operations it is essential that the syringe body be oriented accurately with respect to the setting or filling tube. Since, however, the syringe bodies are often formed of glass, they are gripped in holders with cushioned jaws so that the orientation of their axes cannot be guaranteed to lie on the center axis of the filling or setting tube. The result can be broken or scratched syringe bodies.